Patent Literatures 1 and 2 mentioned below are examples of literature on conventional techniques that are related to a power supply device for an electric discharge machining apparatus.
For a power supply device for an electric discharge machining apparatus described in Patent Literature 1, there is disclosed a technique in which switching elements that interrupt a pulse train having a predetermined repetitive frequency at a predetermined interval are driven to supply AC pulse current to a machining gap between a machining electrode and a workpiece to which a capacitor is connected in parallel, thereby completely discharging the electric charge of the capacitor to the machining gap to prevent arc current from continuously flowing, and thus improve surface roughness during electric discharge machining.
Furthermore, a power supply device for an electric discharge machining apparatus described in Patent Literature 2 is configured to include a DC power supply that supplies DC pulse current to a machining gap between a machining electrode and a workpiece, a series circuit constituted by switching elements and a resistor, a capacitor connected in parallel to the machining electrode and the workpiece, and an electric discharge detector that detects occurrence of electric discharge in the machining gap. For the power supply device for an electric discharge machining apparatus in Patent Literature 2, there is disclosed a technique in which an on/off control is repeated on the switching element, in which the switching element is turned on by a pulse width equal to or smaller than the pulse width of the discharge current of the capacitor, and is turned off for a predetermined time, and repeated until the capacitor discharges and the electric discharge detector detects the electric discharge, and the on/off control is repeated again after a predetermined off-time following the detection of the electric discharge by the electric discharge detector. According to the power supply device for an electric discharge machining apparatus in Patent Literature 2, an electric current having a pulse width equal to or larger than the electric discharge pulse width of the capacitor is not supposed to flow through a gap between the machining electrode and the workpiece (hereinafter, “machining gap”) in order to prevent an arc current from continuously flowing, similarly to Patent Literature 1 (see FIGS. 2 and 11).